The present invention relates to devices for protecting circuit board assemblies from electrostatic discharge.
At present, the electronics industry utilizes a large variety of integrated circuits (chips) which are normally mounted on a circuit board assembly known in the art as a card. Each card usually has a multiplicity of chips mounted thereon.
These integrated circuits are delicate and can become severly damaged if they are exposed to inordinately high voltages, such as static electricity.
A major problem in the electronic industry is that upon handling, packaging or shipping the circuit cards, they become exposed to static electricity from the environment around them. This static electricity if discharged through the chips can damage them since such electrostatic discharge can often be of an inordinately high voltage.
It is common practice to shield the circuit assemblies from electrostatic discharge by enclosing them in insulative packages for shipping purposes. These packages usually are boxes which insulate the card from static electricity or bags such as the static shielding bags, type 2100, produced by the 3M company. The bags produced by 3M act not only as an insulative enclosure but they also have an electricity conductive outer layer which allows static electricity to discharge easily to ground.
However, these devices afford no protection from static discharge once they are removed from the bag.
However, these cards can be exposed to static electricity when they are removed from the shipping container and are handled by, for example, maintenance personnel when they install them. Improper handling may cause electrostatic discharge through the integrated circuits of the circuit board assembly, thus damaging or destroying them.
This electrostatic discharge from an individual can easily be of high voltage as shown in the values below reported by T. S. Speakman in "A MODEL FOR THE FAILURE OF BIPOLAR SILICON INTEGRATED CIRCUITS SUBJECT TO ELECTROSTATIC DISCHARGE", 12th Annual Proc. Reliability Physics, 1974.
______________________________________ Most Common Highest reading Reading ______________________________________ Person walking across carpet 12,000 volts 39,000 volts Person walking across vinyl 4,000 13,000 tile floor Person working at bench 500 3,000 16 Lead DIPS* in plastic box 3,500 12,000 16 Lead DIPS in plastic 500 3,000 shipping tube ______________________________________ *Dual-In-Line Packages Ambient Relative Humidity15% to 36%